Pulp fibers, such as wood pulp fibers, are used in a variety of products including, for example, pulp, paper, paperboard, biofiber composites (e.g., fiber cement board, fiber reinforced plastics, etc.), absorbent products (e.g., fluff pulp, hydrogels, etc.), specialty chemicals derived from cellulose (e.g., cellulose acetate, carboxymethyl cellulose (CMC), etc.), and other products. The pulp fibers can be obtained from a variety of wood types including hardwoods (e.g., oak, gum, maple, poplar, eucalyptus, aspen, birch, etc.), softwoods (e.g., spruce, pine, fir, hemlock, southern pine, redwood, etc.), and non-woods (e.g., kenaf, hemp, straws, bagasse, etc.).
Cellulose nanocrystals (“CNC”) are a relatively new class of material that is made from renewable bio-material feedstock (e.g., lignocellulosic-derived fibers) having remarkable strength, optical, and/or structural properties. CNC are also referred to as nanocrystalline cellulose (“NCC”). While the present application will generally use the terms cellulose nanocrystals or CNC, it should be understood that such terms are used interchangably with nanocrystalline cellulose or NCC. CNC are one of several types of cellulose-based nanoparticles (i.e., particles having at least one dimension less than 100 to 150 nm). Other types of cellulose nanoparticles include nanofibrillated cellulose and tempo-oxidized cellulose.
Compared to other cellulose derived nanoparticles, CNC are distinguished by a relatively high purity of crystalline cellulose, relatively uniform particle size distribution, and the presence of sulfate functional groups bonded to the cellulose molecules. The sulfate groups are the result of the method and conditions of manufacture, and play a key role in determining CNC's unique and potentially valuable properties.
Typically, bleached (or lignin free) pulps are used as feedstock into the reaction that forms CNC. These pulps can be either kraft, soda, or sulfite based pulps, either hardwood or softwood or even non wood pulps, and can be bleached free of lignin in any number of sequences presently practiced in the pulp manufacturing industry.
An exemplary, existing method of manufacturing CNC involves reacting lignocellulosic fibers with aqueous sulfuric acid in the 40 to 60% concentration range at relatively low temperatures (e.g., less than 100° C.), for relatively short and discrete times. The resultant hydrolysis reaction is stopped by dilution and cooling with quench water, which is introduced at a cooler temperature than the reaction mixture. Water will typically be introduced in an amount that is about ten times greater than the amount of reaction mixture in order to slow the reaction. It is important to control the extent of the hydrolysis reaction in order to control the yield and degree of sulfonation to within desired ranges. The extent of the hydrolysis reaction is impacted by the reactant concentrations, temperature, and time, along with the use of a quench dilution/cooling stage.
Typically, the resultant yields are in the range of 35 to 65% with a sulfate content of between 1% and 5% by weight based on the otherwise pure, crystalline cellulose particles. In other words, a significant amount of feedstock (35 to 65% depending on the resultant yield) is dissolved into the aqueous acid phase. The vast majority of sulfuric acid is unreacted. After termination of the hydrolysis reaction by quenching with water, the resultant mixture of crystalline cellulose particles, sulfuric acid, dissolved polysaccharides, other non-polysaccharide materials removed from the feedstock, and water must be separated. The CNC must then be washed, neutralized, and in some cases, dried.
Present methods for recovering CNC involve first separating the (relatively dilute) acid and polysaccharides from the CNC by means of a solid-liquid separation unit operation such as barrier screening, centrifugation, ultra filtration, nanofiltration, and/or dialysis.